During the past 15 years, hydroformylation processes catalyzed by t-phosphine rhodium complexes were widely studied. This effort resulted in the commercial development, by Union Carbide Corporation and Davy McKee Ltd., of a continuous, low pressure hydroformylation process based on a triphenyl phosphine rhodium catalyst system. This system is used in several plants worldwide for the production of n-butyraldehyde by reacting propylene with CO and H.sub.2. The catalysis chemistry of rhodium hydroformylation was recently published by Oswald et al, in the Petroleum Chemistry Division of the American Chemical Society Preprints (Volume 27, Part 2, pages 292 to 309 in March 1982). It is pointed out in the above journal publication that the commercial triphenyl phosphine rhodium catalyst system has a disadvantage in that it is subject to a slow degradation of the triphenyl phosphine ligand.
Hydroformylation processes catalyzed by rhodium and cobalt complexes were discussed in detail and compared in a recent monograph of Juergen Falbe, New Syntheses with Carbon Monoxide, Springer Verlag, New York, 1980. The first chaper, pages 1 to 222, is on "Hydroformylation Oxo Synthesis Roelen Reaction" by B. Cornils. Cornils concludes that present commercial triphenyl phosphine rhodium complex based processes are not only subject to a slow loss of activity but are suitable only for n-aldehyde production when the reactants are 1-n-olefins. For example, they are unsuitable for the commercial production of i-butyraldehyde.
It was disclosed by Morrell and Sherman in U.S. Pat. No. 4,260,828 that the stability of triphenyl phosphine rhodium carbonyl complex hydroformylation catalyst systems could be improved by the addition C.sub.1 to C.sub.4 straight chain unsubstituted alkyl diaryl phosphine ligands. However, Morrell considered stability only in the absence of olefin reactants. He expressly excluded from his invention the use of rhodium and alkyl diaryl phosphine ligands alone without triphenyl phosphine.
The present invention is concerned with branched alkyl diaryl phosphine rhodium complex based catalyst systems containing essentially no complexed triphenyl phosphine ligand. In the present hydroformylation process, the use of such catalyst systems, results in improved thermal and operational stability over that of the triphenyl phosphine rhodium complex system. This allows hydroformylation at increased temperature which is particularly important in a continuous operation in which aldehyde products are removed in the vapor phase.
Further, the alpha and beta-branched alkyl derivatives are additionally distinct in providing a lower n/i ratio of aldehyde products i.e., increased selectivity for iso-aldehydes.
The stability of branched alkyl diaryl phosphine rhodium complex based hydroformylation catalyst systems is improved by increasing the concentration of the excess free alkyl diaryl phosphine ligand. This stability improvement is particularly important in continuous hydroformylation. In general, the increasing excess of the ligand generally decreases the rate of hydroformylation. However, this adverse, inhibitory effect of excess ligand is much smaller in the case of the branched than the normal alkyl diaryl phosphines.